Method of producing finely divided metals



Patented May 20, 1952 Y :reo rares PATENT taoiai-iice fMETfHGDtOFxPROIISIIII FINELYJMVIDED f corporationioflaware Nollrawing. V;21,Plflication'gDiecenibei'. li, 1948, L'Serial No. 625841 :5 T-1 lhegpresentfinvention relates vrtirthe*production ofiilnennetalgowders ,by vftheztherxnal 1deconmcsi- .tloncf rsatmetal "carbonyl for mixtures of different metal fcabonyls.

'The decomposition of :a metal carbonyl fsuch .as the carbonyl 4of iron for nickel or '-'mixtures .theredfis'fdescrbed L'for 'exampleyintu "S, Patents Nos. $11'5936591and 1;'1591661 "and-isusuallyfeiect- .ed by iintroiciuci-1rg the :carbonyl its vaporzed form 1Tinti) `aiheated bvesselfin gsuch fa manner :that

the decomposition takes 4place substantially 1in the free *space Fof 'fthe vessel iinstead of "by Acon-I tactewithffthe heated walls -of =the -vessel. The

metal carbonyl 4decomposcs @with Jthe 5formation y zand @above Girasoli-the Amost:prontiisingr=ap1rlcations lof l-ne- .ly dliuidcd :fmetal Ipowders `lies in 4the 4electronic vfield as :magnetic '-rmaterials. Recent Adevelop- "ments iinffthe:use-f.dffsuch1magnetic-materialslhave shown @that nlbesides :a 'suitable Ivcarbon content, the of ithe findiiiidua'l metal .particles :as fwell .as fthe 3 particle size distribution fof :a mixturefo'f `-auch :particles `.are fo'f :the 4greatest importance vlffor the -iperorniance `electric fdevices, `;particularly in thel'high T`frequency :and iultrae'high vfrequency field. :applications iin the :range fof VVsay 10 to 50 meg. and above, iron particles vhaving ya dia-meter ,3 cto :i4 imicronsioraless- .perform isatis- ."factorfwhereas the f.-performance of particles with an average diameter of .f6 .e038 :micronszis inferioii; Particles with even larger diameters are of little utility for rl'iighffrequency Work.

,As Athe metal carbonyl udecomposition lprincess lhasbreen.ihereto'fore operated `itiinvariablyled to Vmixtures*having 4a large percentage ojoversized particles, 1i. ie marticle sizes having a diameter 0h12 A.microns orabove. Considerable eiortihas heenmadein ',theipast ,toseparate such mixtures off particlesiof widely differents sizes finto `suitable Elections to remove tthe fundesirable -particles aboueeaecertainmaxixnumfsize. JvHowever,nerimprovements have. been devised for-theL decomposition jprocess @itself which Mwould automatically eliminate the Fformation of oversized particles or result 'in powders of a -denite desired particle size. -As'a matterofiact/the-art hadabout concluded^that='the-onlyway'fto obtain uniform-:parvtieles-of Vvthe/desired size -was by the `fractionation jfrnethod.

2I have mow found very surprisingly indeed that the :thermal decomposition of: metal carbonyl can be effected to yield metal :powders :With-1.a closely controlled -particle :size distribution and substantially =free ffrom lparticles above a specilied, fdesired maximum vsize. 'In general, the new -pracess :capable of producing metal powders particularly .suited for application I'in fthe high frequencyvvelectronic.feld Whereicontaminationfof the powder :.withf oversized fractions :must

be avoided.

4The improvement vwhich enables me to vthermally decompose ymetalcarbonyls to yield :metal powders with la controlled vparticle size distribution and vsubstantially Efree from undesired over sized ffractionsifconsists intheistep oilixnitingin the decompositionspaceandunder decomposition conditions, the jContact of -undecomposed fmetal carbonyl `with the'metal fparticles formed in the reactionfto a predetermined time which is'speci'c for 4the desired maximum particle Ysize of `the powder produced and zforithe Vconditions vof temperature, lpressure -and vthroughout -under which lthe decomposition g-reaction is carried out. The

growth of ythe initially formed metal nuclei to particles of, say, 3 to 5 microns in diameter -is assumed -to `be due -to Ythe decomposition of Vthe metal carbonylvon `themetal v*surface of Athe findividualparticles, resulting in "the Vvwell known spherical 'fform -and shell structure Vof carbonyl metal ipowder. By working ras above, 'i. e., ,by limitingiimderdeeornposition'conditions, the contact time of 'th-e ymetal carbonyl with already' formed metal particles to a denite maximum value, iIhave found"that 4the l growth of `the 'individualfparticles Ycan be arrested atpractically any desired :particle size.

ftis'ho'uld be 1noted that Vin the process of Idecomposing metal carbonyls, *the iuse of diluent gasesas wellfasofreducejd*pressurefhave'been ,describedfwhichstepsmay leaclto a decrease in the Ichances Vof collision between undecomposed metall; carbonyl viandalready -formed metal particles. However, no attempts were -made previously '-to=.control `theiparticle size distribution of the metalipowderfformed,f orfto avoid the formation of oversized .ifractions by limiting :the time o'f contact between metal carbonyl and metal powder, which I have recognized'for the iirst time to be the controlling factor in obtaining particle size control. Consequently, by previous operations only inferior powders unsuitable for the requirements of the electronic ultra-high irequency technique were produced. My new process may make use of diluents or reduced pressures for the decomposition reaction, but the adjustment of the proper residence time is the governing factor in avoiding oversized particles.

Thus, if at a certain time of contact, temperature, pressure and diluent concentration, the decomposition leads to oversized particles and an undesired particle size distribution, effective control of the particle size and particle size distribution cannot be attained by any variation in the temperature, pressure or gas concentration so long as the contact time is maintained constant. However, by an appropriate adjustment in the contact time, the control over particle size and their distribution can be effectively realized. It

will, of course, be understood that when an appropriate contact time giving the desired control has been established for any particular temperature, pressure and concentration of gas, the contact time may be varied by variations in the temperature, pressure or concentration of the gas although these alone do not establish the contact time. It is obvious that the limiting, in the decomposition space, of the contact time of metal carbonyl with already formed metal particles can be achieved by several means. Olne of my preferred methods is by maintaining a deiinite rate of decomposition by proper temperature control in the decomposition space', and by'adjusting the throughput of metal carbonyl through the said decomposition space, both conditions depending to some extent on the pressure maintained in the reaction space. Generally the temperature of decomposition will range from 150 to 350 C., the pressure from 2 mm. to 1 atm., the pressure being lower the higher the temperature, and the throughput will be such under such temperature and pressure conditions and depending upon the size of the reactor that the time of contact between the rst formed metal particles and the metal carbonyl shall range from a fraction of a second to not more than 20 seconds. The contact time is measured by the time the metal carbonyl or its decomposition products are in the reactor under decomposition conditions, i. e., at the temperature at which decomposition occurs.

The metal carbonyl may .be diluted with inert gases, such as ammonia, nitrogen, hydrogen and car-bon monoxide to facilitate the control, the range of the dilution being such that there is present from about 10 to 75% of metal carbonyl.

It will be understood that my new process for obtaining metal powders of deiinite desired particle size as above. involves the complete decomposition, in one pass, of the metal carbonyl .entering the reaction space. However, the process may be carried out in such a way that undecomposed carbonyl is left in the reaction mixture leaving the decomposition space. In the latter operation, provisions are made at the end of the time allowed for the contact between metal carbonyl and already formed metal particles to arrest the decomposition reaction, for example, by quenching of the hot reaction mixture with cold liquids or cold gases. The liquids or gases employed are inert to the reactants and may be silicon oils such as are described in U. S. Patents Nos.

2,258,218, 2,258,220 and 2,258,222, or hydrocarbon oils, such as kerosene or gasoline, ammonia, nitrogen, hydrogen or carbon monoxide. The liquids and gases will be cooled below the reaction temperature, say, from about C. to room temperature. i

The reaction may alsobe arrested by removal of the metal particles grown to the desired size from the reaction mixture by means of magnets surrounding the wall of the reactor at a point along the path of'travel of the gases, such that the desired time of contact between initially formed carbonyl metal and metal carbonyl will have elapsed when the magnets are reached.

Examples of metal carbonyls to which my process is applicable are iron, nickel, cobalt and molybdenum. The metal carbonyls may be used along or in admixture with each other, such as a mixture of nickel and iron carbonyl, a mixture of cobalt and molybdenum carbonyl, and the like.

An illustration of the results which may be achieved by following my procedure is aiorded by the following. When decomposing iron carbonyl vapor in the free space of a heated cylindrical vessel at a temperature of 280 C., under an absolute pressure of 1 atm., and at such a throughput that a contact time in the heated reactor` space of iron carbonyl vapor with metal particles of approximately A240 seconds is achieved, a metal powder is obtained with a weight average particle diameter of 7 to 8 microns, containing oversized particles amounting to approximately 15% by weight of the powder and having a diameter of 12 microns and above.

However, when according to the present invention the reaction conditions of temperature, pres- `1re and throughout are adjusted in such a manner that a rapid decomposition of the metal carbonyl is achieved and the contact of the metal carbonyl with already formed iron particles is limited to a maximum time of 10 seconds, a-metal powder is obtained with a weight average particle diameter of 4 to 5 microns. This powder is substantially free from particles having a diameter of 9 microns or more.

It can be clearly seen that by performing the decomposition reaction at a very rapid rate and limiting, under suitable conditions of temperature, pressure and throughput, the time of contact between metal carbonyl and already formed metal particles, i. e., to one second or less, particles having a weight average diameter of 2 microns, 1 micron, or less than 1 micron can be produced. An essentially instantaneous decomposition which can be achieved under properly adjusted conditions, i. e., a temperature of 300 C. and a. vacuum of 20 mm. mercury, results in the formation of an extremely fine metal powder consisting of particles of less than 1/10 of a micron in diameter and substantially free from any over sized material.

The following example will further illustrate the nature of this invention, but the invention is not restricted to this example.

Example Iron penta carbonyl is evaporated in a steam heated evaporator and the vapor led at the rate of 1 cu. ft. per minute at atmospheric pressure into the free space of a cylindrical vessel 16 ft. high and having a diameter of 3 ft. and maintained by means of a vacuum pump at an absolute pressure of 30 mm. mercury. The vessel is heated by means of a hot gas circulating through a jacket surrounding the decomposition chamber.

The temperature inside the decomposition vessel is thus held at 250 C. The carbonyl vapor entering the reaction zone continuously from the top is decomposed, by radiant heat, within 2 to 3 seconde, into nely divided iron particles and carbon monoxide gas. The reaction mixture is continuously withdrawn from the bottom of the decomposition chamber and separated by means of a separating tank and Iilter, yielding iron particles with an average diameter of 2-3 microns, and substantially free from particles above 6 microns in diameter.

If in the above reaction the decomposition time, i. e., the time of contact between iron carbonyl and already formed metal particles is increased to 8-9 seconds, for example. by decreasing the temperature in the reactor to 220 C. and increasing the pressure to 100 mm. mercury, a metal powder is obtained with an average particle diameter of 5-6 microns.

I claim:

1. In the production of flne metal powders by thermally decomposing a metal carbonyl, the step of controlling substantially all of the distribution of the average particle size within a range of about 2 microns and the maximum particle size to a small amount above the average, said maximum size being no more than about 3 to 4 microns larger than the average, which particles are produced by limiting the contact in the decomposition space of undecomposed metal carbonyl with the metal particles formed in the reaction to a denite length of time not exceeding 20 seconds specifically for the desired average and maximum particle size of the powder produced at a reaction temperature of G-350 C. and a pressure from mm. to 100 mm. absolute.

2. The process as dened in claim 1 in which the decomposition of the metal carbonyl is car- 6 ried out under such operating conditions that undecomposed metal carbonyl is withdrawn from the reaction space, together with the decomposition products, and the separation of the metal powder produced is subsequently effected.

3. The process as defined in claim 1 wherein the metal carbonyl is iron carbonyl and wherein the temperature, pressure and throughput are adjusted within the stated range so that decomposition is substantially complete in less than 10 seconds.

4. The process as dened in claim 1 wherein the metal carbonyl is iron carbonyl and wherein the temperature, pressure and throughput are adjusted within the stated range so that the decomposition is substantially complete in less than ve seconds.

5. The process as defined in claim 1 wherein the metal carbonyl is iron carbonyl and wherein the temperature, pressure and throughput are adjusted within the stated range so that decomposition is essentially instantaneous.

HANS BELLER.

REFERENCES CITED The following references are of record in the ille of this patent:

UNITED STATES PATENTS Number Name Date 1,759,661 Muller et al May 20, 1930 1,836,732 Schlecht et al Dec. 15, 1931 2,041,480 Oexmann May 19. 1936 OTHER REFERENCES Symposium on Powder Metallurgy, Second edition. published by The Iron and Steel Institute, London, December 1947, page 49. 

1. IN THE PRODUCTION OF FINE METAL POWDERS BY THERMALLY DECOMPOSING A METAL CARBONYL, THE STEP OF CONTROLLING SUBSTANTIALLY ALL OF THE DISTRIBUTION OF THE AVERAGE PARTICLE SIZE WITHIN A RANGE OF ABOUT 2 MICRONS AND THE MAXIMUM PARTICLE SIZE TO A SMALL AMOUNT ABOVE THE AVERAGE, SAID MAXIMUM SIZE BEING NO MORE THAN ABOUT 3 TO 4 MICRONS LARGER THAN THE AVERAGE, WHICH PARTICLES ARE PRODUCED BY LIMITING THE CONTACT IN THE DECOMPOSITION SPACE OF UNDECOMPOSED METAL CARBONYL WITH THE METAL PARTICLES FORMED IN THE REACTION TO A DEFINITE LENGTH OF TIME NOT EXCEEDING 20 SECONDS SPECIFICALLY FOR THE DESIRED AVERAGE AND MAXIMUM PARTICLE SIZE OF THE POWDER PRODUCED AT A REACTION TEMPERATURE OF 150-350* C. AND A PRESSURE FROM 20 MM. TO 100 MM. ABSOLUTE. 